Fluid Film Bearings (FFBs) and Active Magnetic Bearings (AMBs) are competing devices in the marketplace. FFBs, in particular Journal Bearings (JBs), are superior load carrying elements due to their larger load carrying capacity and their ability to introduce passive damping to the rotor system. However, JBs exhibit an unstable vibration at high speed, called oil whip, which is excited when the rotor speed reaches about twice the first critical speed. This instability limits the possibility of increasing the rotor speed of rotation.
AMBs, on the other hand, provide contactless rotor support, at high speeds and are free from whip instabilities. They have an added feature which is their capabilities as controlling elements. AMBs can provide variable and controllable stiffness and damping and additionally can provide unbalance control and many other control features. However, AMBs suffer from certain shortcomings. In particular reliability issues are always a concern for AMB designers. In fact, AMBs are always designed with a redundant bearing system, called a “backup bearing”, such that the backup bearing carries the rotor in case of AMB failure.